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coding_drills / art_jYW0iSLZW84
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{"kind": "coding_drills", "role_id": "role_wxdcPTRw9aQ", "source": "leetcode_graphql", "generated_at": "2026-06-20T04:51:42.048235+00:00", "difficulty_mix": {"Easy": 3, "Medium": 5, "Hard": 2}, "topics_filter": [], "questions": [{"index": 0, "category": "coding_string", "question": "[Easy] LC #20 \u00b7 Valid Parentheses \u2014 solve on leetcode.com", "why": "Topic tags: string, stack. AC rate 44.2%. Open the editor at https://leetcode.com/problems/valid-parentheses/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "valid-parentheses", "frontend_id": "20", "difficulty": "Easy", "ac_rate": 44.1741531650066, "topics": ["string", "stack"], "url": "https://leetcode.com/problems/valid-parentheses/", "problem_html": "<p>Given a string <code>s</code> containing just the characters <code>'('</code>, <code>')'</code>, <code>'{'</code>, <code>'}'</code>, <code>'['</code> and <code>']'</code>, determine if the input string is valid.</p>\n\n<p>An input string is valid if:</p>\n\n<ol>\n\t<li>Open brackets must be closed by the same type of brackets.</li>\n\t<li>Open brackets must be closed in the correct order.</li>\n\t<li>Every close bracket has a corresponding open bracket of the same type.</li>\n</ol>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">s = "()"</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">true</span></p>\n</div>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">s = "()[]{}"</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">true</span></p>\n</div>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">s = "(]"</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">false</span></p>\n</div>\n\n<p><strong class=\"example\">Example 4:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">s = "([])"</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">true</span></p>\n</div>\n\n<p><strong class=\"example\">Example 5:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">s = "([)]"</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">false</span></p>\n</div>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= s.length <= 10<sup>4</sup></code></li>\n\t<li><code>s</code> consists of parentheses only <code>'()[]{}'</code>.</li>\n</ul>\n", "hints": ["Use a stack of characters.", "When you encounter an opening bracket, push it to the top of the stack.", "When you encounter a closing bracket, check if the top of the stack was the opening for it. If yes, pop it from the stack. Otherwise, return false."]}}, {"index": 1, "category": "coding_two_pointers", "question": "[Easy] LC #28 \u00b7 Find the Index of the First Occurrence in a String \u2014 solve on leetcode.com", "why": "Topic tags: two-pointers, string, string-matching. AC rate 46.6%. Open the editor at https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "find-the-index-of-the-first-occurrence-in-a-string", "frontend_id": "28", "difficulty": "Easy", "ac_rate": 46.646679248066825, "topics": ["two-pointers", "string", "string-matching"], "url": "https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/", "problem_html": "<p>Given two strings <code>needle</code> and <code>haystack</code>, return the index of the first occurrence of <code>needle</code> in <code>haystack</code>, or <code>-1</code> if <code>needle</code> is not part of <code>haystack</code>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> haystack = "sadbutsad", needle = "sad"\n<strong>Output:</strong> 0\n<strong>Explanation:</strong> "sad" occurs at index 0 and 6.\nThe first occurrence is at index 0, so we return 0.\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> haystack = "leetcode", needle = "leeto"\n<strong>Output:</strong> -1\n<strong>Explanation:</strong> "leeto" did not occur in "leetcode", so we return -1.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= haystack.length, needle.length <= 10<sup>4</sup></code></li>\n\t<li><code>haystack</code> and <code>needle</code> consist of only lowercase English characters.</li>\n</ul>\n", "hints": []}}, {"index": 2, "category": "coding_array", "question": "[Easy] LC #35 \u00b7 Search Insert Position \u2014 solve on leetcode.com", "why": "Topic tags: array, binary-search. AC rate 51.3%. Open the editor at https://leetcode.com/problems/search-insert-position/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "search-insert-position", "frontend_id": "35", "difficulty": "Easy", "ac_rate": 51.25739858897139, "topics": ["array", "binary-search"], "url": "https://leetcode.com/problems/search-insert-position/", "problem_html": "<p>Given a sorted array of distinct integers and a target value, return the index if the target is found. If not, return the index where it would be if it were inserted in order.</p>\n\n<p>You must write an algorithm with <code>O(log n)</code> runtime complexity.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [1,3,5,6], target = 5\n<strong>Output:</strong> 2\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [1,3,5,6], target = 2\n<strong>Output:</strong> 1\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [1,3,5,6], target = 7\n<strong>Output:</strong> 4\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= nums.length <= 10<sup>4</sup></code></li>\n\t<li><code>-10<sup>4</sup> <= nums[i] <= 10<sup>4</sup></code></li>\n\t<li><code>nums</code> contains <strong>distinct</strong> values sorted in <strong>ascending</strong> order.</li>\n\t<li><code>-10<sup>4</sup> <= target <= 10<sup>4</sup></code></li>\n</ul>\n", "hints": []}}, {"index": 3, "category": "coding_linked_list", "question": "[Medium] LC #2 \u00b7 Add Two Numbers \u2014 solve on leetcode.com", "why": "Topic tags: linked-list, math, recursion. AC rate 48.5%. Open the editor at https://leetcode.com/problems/add-two-numbers/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "add-two-numbers", "frontend_id": "2", "difficulty": "Medium", "ac_rate": 48.46758391189911, "topics": ["linked-list", "math", "recursion"], "url": "https://leetcode.com/problems/add-two-numbers/", "problem_html": "<p>You are given two <strong>non-empty</strong> linked lists representing two non-negative integers. The digits are stored in <strong>reverse order</strong>, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.</p>\n\n<p>You may assume the two numbers do not contain any leading zero, except the number 0 itself.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg\" style=\"width: 483px; height: 342px;\" />\n<pre>\n<strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]\n<strong>Output:</strong> [7,0,8]\n<strong>Explanation:</strong> 342 + 465 = 807.\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> l1 = [0], l2 = [0]\n<strong>Output:</strong> [0]\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]\n<strong>Output:</strong> [8,9,9,9,0,0,0,1]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of nodes in each linked list is in the range <code>[1, 100]</code>.</li>\n\t<li><code>0 <= Node.val <= 9</code></li>\n\t<li>It is guaranteed that the list represents a number that does not have leading zeros.</li>\n</ul>\n", "hints": []}}, {"index": 4, "category": "coding_array", "question": "[Medium] LC #48 \u00b7 Rotate Image \u2014 solve on leetcode.com", "why": "Topic tags: array, math, matrix. AC rate 80.1%. Open the editor at https://leetcode.com/problems/rotate-image/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "rotate-image", "frontend_id": "48", "difficulty": "Medium", "ac_rate": 80.08370211767836, "topics": ["array", "math", "matrix"], "url": "https://leetcode.com/problems/rotate-image/", "problem_html": "<p>You are given an <code>n x n</code> 2D <code>matrix</code> representing an image, rotate the image by <strong>90</strong> degrees (clockwise).</p>\n\n<p>You have to rotate the image <a href=\"https://en.wikipedia.org/wiki/In-place_algorithm\" target=\"_blank\"><strong>in-place</strong></a>, which means you have to modify the input 2D matrix directly. <strong>DO NOT</strong> allocate another 2D matrix and do the rotation.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/08/28/mat1.jpg\" style=\"width: 500px; height: 188px;\" />\n<pre>\n<strong>Input:</strong> matrix = [[1,2,3],[4,5,6],[7,8,9]]\n<strong>Output:</strong> [[7,4,1],[8,5,2],[9,6,3]]\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/08/28/mat2.jpg\" style=\"width: 500px; height: 201px;\" />\n<pre>\n<strong>Input:</strong> matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]\n<strong>Output:</strong> [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>n == matrix.length == matrix[i].length</code></li>\n\t<li><code>1 <= n <= 20</code></li>\n\t<li><code>-1000 <= matrix[i][j] <= 1000</code></li>\n</ul>\n", "hints": []}}, {"index": 5, "category": "coding_array", "question": "[Medium] LC #36 \u00b7 Valid Sudoku \u2014 solve on leetcode.com", "why": "Topic tags: array, hash-table, matrix. AC rate 64.5%. Open the editor at https://leetcode.com/problems/valid-sudoku/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "valid-sudoku", "frontend_id": "36", "difficulty": "Medium", "ac_rate": 64.51813707093162, "topics": ["array", "hash-table", "matrix"], "url": "https://leetcode.com/problems/valid-sudoku/", "problem_html": "<p>Determine if a <code>9 x 9</code> Sudoku board is valid. Only the filled cells need to be validated <strong>according to the following rules</strong>:</p>\n\n<ol>\n\t<li>Each row must contain the digits <code>1-9</code> without repetition.</li>\n\t<li>Each column must contain the digits <code>1-9</code> without repetition.</li>\n\t<li>Each of the nine <code>3 x 3</code> sub-boxes of the grid must contain the digits <code>1-9</code> without repetition.</li>\n</ol>\n\n<p><strong>Note:</strong></p>\n\n<ul>\n\t<li>A Sudoku board (partially filled) could be valid but is not necessarily solvable.</li>\n\t<li>Only the filled cells need to be validated according to the mentioned rules.</li>\n</ul>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img src=\"https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png\" style=\"height:250px; width:250px\" />\n<pre>\n<strong>Input:</strong> board = \n[["5","3",".",".","7",".",".",".","."]\n,["6",".",".","1","9","5",".",".","."]\n,[".","9","8",".",".",".",".","6","."]\n,["8",".",".",".","6",".",".",".","3"]\n,["4",".",".","8",".","3",".",".","1"]\n,["7",".",".",".","2",".",".",".","6"]\n,[".","6",".",".",".",".","2","8","."]\n,[".",".",".","4","1","9",".",".","5"]\n,[".",".",".",".","8",".",".","7","9"]]\n<strong>Output:</strong> true\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> board = \n[["8","3",".",".","7",".",".",".","."]\n,["6",".",".","1","9","5",".",".","."]\n,[".","9","8",".",".",".",".","6","."]\n,["8",".",".",".","6",".",".",".","3"]\n,["4",".",".","8",".","3",".",".","1"]\n,["7",".",".",".","2",".",".",".","6"]\n,[".","6",".",".",".",".","2","8","."]\n,[".",".",".","4","1","9",".",".","5"]\n,[".",".",".",".","8",".",".","7","9"]]\n<strong>Output:</strong> false\n<strong>Explanation:</strong> Same as Example 1, except with the <strong>5</strong> in the top left corner being modified to <strong>8</strong>. Since there are two 8's in the top left 3x3 sub-box, it is invalid.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>board.length == 9</code></li>\n\t<li><code>board[i].length == 9</code></li>\n\t<li><code>board[i][j]</code> is a digit <code>1-9</code> or <code>'.'</code>.</li>\n</ul>\n", "hints": []}}, {"index": 6, "category": "coding_string", "question": "[Medium] LC #38 \u00b7 Count and Say \u2014 solve on leetcode.com", "why": "Topic tags: string. AC rate 62.9%. Open the editor at https://leetcode.com/problems/count-and-say/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "count-and-say", "frontend_id": "38", "difficulty": "Medium", "ac_rate": 62.880196768473894, "topics": ["string"], "url": "https://leetcode.com/problems/count-and-say/", "problem_html": "<p>The <strong>count-and-say</strong> sequence is a sequence of digit strings defined by the recursive formula:</p>\n\n<ul>\n\t<li><code>countAndSay(1) = "1"</code></li>\n\t<li><code>countAndSay(n)</code> is the run-length encoding of <code>countAndSay(n - 1)</code>.</li>\n</ul>\n\n<p><a href=\"http://en.wikipedia.org/wiki/Run-length_encoding\" target=\"_blank\">Run-length encoding</a> (RLE) is a string compression method that works by replacing consecutive identical characters (repeated 2 or more times) with the concatenation of the character and the number marking the count of the characters (length of the run). For example, to compress the string <code>"3322251"</code> we replace <code>"33"</code> with <code>"23"</code>, replace <code>"222"</code> with <code>"32"</code>, replace <code>"5"</code> with <code>"15"</code> and replace <code>"1"</code> with <code>"11"</code>. Thus the compressed string becomes <code>"23321511"</code>.</p>\n\n<p>Given a positive integer <code>n</code>, return <em>the </em><code>n<sup>th</sup></code><em> element of the <strong>count-and-say</strong> sequence</em>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">n = 4</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">"1211"</span></p>\n\n<p><strong>Explanation:</strong></p>\n\n<pre>\ncountAndSay(1) = "1"\ncountAndSay(2) = RLE of "1" = "11"\ncountAndSay(3) = RLE of "11" = "21"\ncountAndSay(4) = RLE of "21" = "1211"\n</pre>\n</div>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">n = 1</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">"1"</span></p>\n\n<p><strong>Explanation:</strong></p>\n\n<p>This is the base case.</p>\n</div>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= n <= 30</code></li>\n</ul>\n\n<p> </p>\n<strong>Follow up:</strong> Could you solve it iteratively?", "hints": ["Create a helper function that maps an integer to pairs of its digits and their frequencies. For example, if you call this function with \"223314444411\", then it maps it to an array of pairs [[2,2], [3,2], [1,1], [4,5], [1, 2]].", "Create another helper function that takes the array of pairs and creates a new integer. For example, if you call this function with [[2,2], [3,2], [1,1], [4,5], [1, 2]], it should create \"22\"+\"23\"+\"11\"+\"54\"+\"21\" = \"2223115421\".", "Now, with the two helper functions, you can start with \"1\" and call the two functions alternatively n-1 times. The answer is the last integer you will obtain."]}}, {"index": 7, "category": "coding_string", "question": "[Medium] LC #22 \u00b7 Generate Parentheses \u2014 solve on leetcode.com", "why": "Topic tags: string, dynamic-programming, backtracking. AC rate 78.7%. Open the editor at https://leetcode.com/problems/generate-parentheses/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "generate-parentheses", "frontend_id": "22", "difficulty": "Medium", "ac_rate": 78.66010549076367, "topics": ["string", "dynamic-programming", "backtracking"], "url": "https://leetcode.com/problems/generate-parentheses/", "problem_html": "<p>Given <code>n</code> pairs of parentheses, write a function to <em>generate all combinations of well-formed parentheses</em>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<pre><strong>Input:</strong> n = 3\n<strong>Output:</strong> [\"((()))\",\"(()())\",\"(())()\",\"()(())\",\"()()()\"]\n</pre><p><strong class=\"example\">Example 2:</strong></p>\n<pre><strong>Input:</strong> n = 1\n<strong>Output:</strong> [\"()\"]\n</pre>\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= n <= 8</code></li>\n</ul>\n", "hints": []}}, {"index": 8, "category": "coding_array", "question": "[Hard] LC #37 \u00b7 Sudoku Solver \u2014 solve on leetcode.com", "why": "Topic tags: array, hash-table, backtracking, matrix. AC rate 65.5%. Open the editor at https://leetcode.com/problems/sudoku-solver/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "sudoku-solver", "frontend_id": "37", "difficulty": "Hard", "ac_rate": 65.47220302782839, "topics": ["array", "hash-table", "backtracking", "matrix"], "url": "https://leetcode.com/problems/sudoku-solver/", "problem_html": "<p>Write a program to solve a Sudoku puzzle by filling the empty cells.</p>\n\n<p>A sudoku solution must satisfy <strong>all of the following rules</strong>:</p>\n\n<ol>\n\t<li>Each of the digits <code>1-9</code> must occur exactly once in each row.</li>\n\t<li>Each of the digits <code>1-9</code> must occur exactly once in each column.</li>\n\t<li>Each of the digits <code>1-9</code> must occur exactly once in each of the 9 <code>3x3</code> sub-boxes of the grid.</li>\n</ol>\n\n<p>The <code>'.'</code> character indicates empty cells.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img src=\"https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png\" style=\"height:250px; width:250px\" />\n<pre>\n<strong>Input:</strong> board = [["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]\n<strong>Output:</strong> [["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]\n<strong>Explanation:</strong> The input board is shown above and the only valid solution is shown below:\n\n<img src=\"https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sudoku-by-L2G-20050714_solution.svg/250px-Sudoku-by-L2G-20050714_solution.svg.png\" style=\"height:250px; width:250px\" />\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>board.length == 9</code></li>\n\t<li><code>board[i].length == 9</code></li>\n\t<li><code>board[i][j]</code> is a digit or <code>'.'</code>.</li>\n\t<li>It is <strong>guaranteed</strong> that the input board has only one solution.</li>\n</ul>\n", "hints": ["For each cell, place a valid number and try solving for the remaining empty cells.", "If stuck, undo (backtrack) and try another valid number."]}}, {"index": 9, "category": "coding_linked_list", "question": "[Hard] LC #25 \u00b7 Reverse Nodes in k-Group \u2014 solve on leetcode.com", "why": "Topic tags: linked-list, recursion. AC rate 66.1%. Open the editor at https://leetcode.com/problems/reverse-nodes-in-k-group/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "reverse-nodes-in-k-group", "frontend_id": "25", "difficulty": "Hard", "ac_rate": 66.08188830002686, "topics": ["linked-list", "recursion"], "url": "https://leetcode.com/problems/reverse-nodes-in-k-group/", "problem_html": "<p>Given the <code>head</code> of a linked list, reverse the nodes of the list <code>k</code> at a time, and return <em>the modified list</em>.</p>\n\n<p><code>k</code> is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of <code>k</code> then left-out nodes, in the end, should remain as it is.</p>\n\n<p>You may not alter the values in the list's nodes, only nodes themselves may be changed.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg\" style=\"width: 542px; height: 222px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,3,4,5], k = 2\n<strong>Output:</strong> [2,1,4,3,5]\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg\" style=\"width: 542px; height: 222px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,3,4,5], k = 3\n<strong>Output:</strong> [3,2,1,4,5]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of nodes in the list is <code>n</code>.</li>\n\t<li><code>1 <= k <= n <= 5000</code></li>\n\t<li><code>0 <= Node.val <= 1000</code></li>\n</ul>\n\n<p> </p>\n<p><strong>Follow-up:</strong> Can you solve the problem in <code>O(1)</code> extra memory space?</p>\n", "hints": []}}]}